A government intelligence expert said yesterday that it was "a reasonable assumption" that the Soviet Union used sophisticated machinery purchased from the United States six years ago to develop the new, accurate guidance system in its SS18, the world's largest and most destructive intercontinental ballistic missile.

Dr. Jack Vorona, assistant vice director for scientific and technical intelligence of the Defense Intelligence Agency, also told a Senate Armed Services subcommittee that:

At least some of the trucks produced by a Soviet plant built with U.S. technology are being used by the military and the plant's excess engines may be destined for other military vehicles.

Illegally diverted IBM 360 and 370 computers may have been the cornerstone for a computer system that directs Warsaw Pact air defense.

A student exchange program permitted the Soviets to send one young engineer to the United States to study "fuel-air explosives" -- a particularly large-blast weapon system. The man, Vorona testified, is now involved in the Soviet fuel-air explosive program.

Vorona's testimony may signal a new attempt to curtail commerical sales sharply and limit academic exchanges with the soviet by government officials who believe the United States should take a harder line with Moscow.

Sen. Harry F. Byrd Jr. (Ind-Va.), who chaired the hearing, asked Vorona, "Would it be accurate to say American knowhow is being used to develop the Soviet war machine?"

"Yes sir, very accurate," the witness replied.

That view, however, is not held throughout the government, as battles within various administrations over the past 20 years have shown.

By law, the Commerce Department must license sales off certain goods to the Soviet Union in a process that has been the focus of continuing controversy.

In 1973 and 1974, for example, the Pentagon opposed, but Commerce and the White House finally approved the sale of 164 precision miniature ball bearing grinding machines.

Those machines, Vorona said yesterday, "are making a distinctive contribution to the Soviet military effort and could very well be producing the precision minature ball bearings used in current and follow-on high quality" guidance systems aboard Soviet ballistic missiles.

Vorona touched another matter of controversy involving Soviet trade by calling attention to "an Austrian firm" which is the world's leading producer of equipment used to make high-quality artillery tubes.

The Soviets, he said, have purchased "probably hundreds of millions of dollars worth" of this company's machinery, and now "have the greatest gun-barrel manufacturing capability in the world."

One of Vorona's charges -- that the Soviet-U.S. student exchange program "has been detrimental to the U.S." -- was challenged in part yesterdy by an official involved in the program who asked to remain anonymous.

Vorona said some Soviet students, by previous training the equals of U.S. PhDs, have involved themselves in projects in "hard sciences," such as advanced electronics and computer technology.

Conversely, Vorona said, American students going to the Soviet Union tend to concentrate on "langugage, history, social sciences."

Vorona said he doubts the Soviets would permit exchange students to do work in higher mathematics, laser technology and other advanced fields with potential military applications.

However, an official of the National Academy of Sciences told the Washington Post that the Soviets have granted study visas to "an unusually high number of [U.S.] mathematicians."